Wireless communication has become an integral part of modern life in personal and professional realms. It is used for voice, data, and other types of communication. Wireless communication is also used in military and emergency response applications. Communications that are made wirelessly rely on the electromagnetic spectrum as the carrier medium. Unfortunately, the electromagnetic spectrum is a limited resource.
Although the electromagnetic spectrum spans a wide range of frequencies, only certain frequency bands are applicable for certain uses due to their physical nature and/or due to governmental restrictions. Moreover, the use of the electromagnetic spectrum for wireless communications is so pervasive that many frequency bands are already over-crowded. This crowding may cause interference between and among different wireless communication systems.
Such interference jeopardizes successful transmission and reception of wireless communications that are important to many different aspects of modern society. Wireless communication interference can necessitate retransmissions, cause the use of ever greater power outlays, or even completely prevent some wireless communications. Consequently, there is a need to wirelessly communicate in the presence of electromagnetic interference that may otherwise hinder the successful communication of information.
Multipath fading results in reduced communications reliability, particularly where mobile devices pass through signal fades. Linearly polarized communication systems are generally more susceptible to multipath fading than elliptically or circularly polarized systems. Various mobile and stationary wireless systems often employ an omni-directional antenna pattern on their receivers and/or transmitters. An omni-directional antenna is characterized by an azimuthal radiation pattern that exhibits minimal antenna gain variation. Dual polarized (D-pol) omni-directional antennas allow for an increase in data throughput by exploiting nominally orthogonal vertical and horizontal polarizations associated with individual respective vertical and horizontal channels “Vertical” and “horizontal” are commonly used as a convenient shorthand for any two nominally orthogonal polarization states.
However, due to the nature of systems having D-pol omni-directional antennas, the relative orientation of the vertical and horizontal polarizations between transmit and receive antennas may vary based on movement within a mobile system, and/or other inherent sources of transmit-receive antenna polarization misalignment, such as polarization scattering in a communication path, fading differences in vertical and horizontal signal components, etc. Additionally, the relative orientation of the vertical and horizontal polarizations in some antennas may be modified electronically, such as in adaptive circularly and elliptically polarized antenna systems.
Non-equal polarization is commonly defined by two or more polarization states separated from each other on the Poincaré Sphere. In contrast, exactly orthogonal polarization is defined by two polarization states separated exactly by 180 degrees on the Poincaré Sphere. Additionally, nominally orthogonal polarization is defined by two or more polarization states that may deviate from being exactly orthogonal based on standard commercial manufacturing variations or tolerances.
There are a number of existing methods that address polarization mismatch between a transmitter and a receiver as well as multipath fading. For example, spatial diversity uses two or more antennas separated in space, thereby experiencing differing fading environments. Polarization diversity uses two or more antennas exhibiting differing polarization states. These two diversity techniques can take on various implementations. For example, a technique referred to as switched diversity selects one of the antennas that exhibits the best quality metric. Maximum Ratio Combining (MRC) combines the outputs of all antennas simultaneously to maximize the Signal to Noise Ratio (SNR). Minimum Mean Square Error (MMSE) combining, like MRC, makes use of one or more antenna and can maximize a Signal to Interference plus Noise Ratio (SINR). However, MMSE requires carrier recovery as an integral component of the algorithm, so it is limited to coherent communication systems. MMSC cannot be used to optimize the SINR of signals with unknown carrier phase if the initial SINR is too low to allow carrier recovery.
Many wireless systems, such as various mobile systems, automated meter reading (AMR) installations and advanced metering infrastructure (AMI) installations, often include non-coherent systems that do not support carrier recovery, as well as coherent systems that require carrier recovery. Wireless systems may also exploit a Time Division Duplex (TDD) scheme that requires only one channel for transmitting downlink (i.e., forward link) and receiving uplink (i.e., reverse link) sub-frames at two distinct time slots. Techniques may be employed, for example, at concentrators and/or repeaters that maximize SINR of a received uplink signal from an endpoint via use of an Adaptive Polarization Array (APA) and suitable signal processing capabilities. However, due to cost constraints, endpoints may not be configured with APA and/or other suitable signal processing capabilities. For example, in AMR and AMI systems, the polarization of a given endpoint's receiver and transmitter is commonly fixed. Therefore, there is a need for entities, such as concentrators and repeaters, to provide a downlink signal with a polarization that optimizes SINR at a receiving endpoint.